Enable linguist for our build repo (#390)

Based on https://docs.github.com/en/github/administering-a-repository/customizing-how-changed-files-appear-on-github
this should be useful to keep certain files from displaying in diffs by default, or counting
toward the repository language.

This should help when PRs with vendor folders or generated clients are adding/removing changes.

.gitattributes

@@ -0,0 +1,6 @@
+# This file is documented at https://git-scm.com/docs/gitattributes.
+# Linguist-specific attributes are documented at
+# https://github.com/github/linguist.
+
+**/zz_generated.*.go linguist-generated=true
+/pkg/clients/** linguist-generated=true